Thank you so much! I hope you have fun with it sometime~
The card battle system was a real challenge that never got properly solved, and I'm sure there's better and more efficient ways to do it. The cards (which are kept on background-style image sheets) each have their own variable, plus a bunch more for game stats (damage, characters, abilities, etc.). I used the swap tiles plugin to change the card images in-game, which limited the total number of cards I could include. All of the variables are reset and randomized when you start the game/each match. The rest is just a lot of toggling of variables in-game and math functions to make the cards link together and do the right amounts of damage!
There are official card games on the original Game Boy, like an early Yu-Gi-Oh!, that have way more cards than what I was able to include, but I don't know how they managed to do it. ^^;